The pancreas

The pancreas consists of clusters if endocrine cells (the islets of Langerhans) and exocrine cells whose secretions drain into the duodenum.

Pancreatic fluid contains:

• sodium bicarbonate (NaHCO₃). This neutralizes the acidity of the fluid arriving from the stomach raising its pH to about 8.
• pancreatic amylase. This enzyme hydrolyzes starch into a mixture of maltose and glucose.
• pancreatic lipase. The enzyme hydrolyzes ingested fats into a mixture of fatty acids and monoglycerides. Its action is enhanced by the detergent effect of bile.
• 4 zymogens— proteins that are precursors to active proteases. These are immediately converted into the active proteolytic enzymes:
  • trypsin. Trypsin cleaves peptide bonds on the C-terminal side of arginines and lysines.
  • chymotrypsin. Chymotrypsin cuts on the C-terminal side of tyrosine, phenylalanine, and tryptophan residues (the same bonds as pepsin, whose action ceases when the NaHCO₃ raises the pH of the intestinal contents).
  • elastase. Elastase cuts peptide bonds next to small, uncharged side chains such as those of alanine and serine.
  • carboxypeptidase. This enzyme removes, one by one, the amino acids at the C-terminal of peptides.
• nucleases. These hydrolyze ingested nucleic acids (RNA and DNA) into their component nucleotides.

The secretion of pancreatic fluid is controlled by two hormones:

• secretin, which mainly affects the release of sodium bicarbonate, and
• cholecystokinin (CCK), which stimulates the release of the digestive enzymes.

Vital Capacity: The vital capacity (VC) is the maximum volume which can be ventilated in a single breath. VC= IRV+TV+ERV. VC varies with gender, age, and body build. Measuring VC gives a device for diagnosis of respiratory disorder, and a benchmark for judging the effectiveness of treatment. (4600 ml)

Vital Capacity is reduced in restrictive disorders, but not in disorders which are purely obstructive.

The FEV₁ is the % of the vital capacity which is expelled in the first second. It should be at least 75%. The FEV₁ is reduced in obstructive disorders.

Both VC and the FEV₁ are reduced in disorders which are both restrictive and obstructive

Oxygen is present at nearly 21% of ambient air. Multiplying .21 times 760 mmHg (standard pressure at sea level) yields a pO₂ of about 160. Carbon dioxide is .04% of air and its partial pressure, pCO₂, is .3.

With alveolar air having a pO₂ of 104 and a pCO₂ of 40. So oxygen diffuses into the alveoli from inspired air and carbon dioxide diffuses from the alveoli into air which will be expired. This causes the levels of oxygen and carbon dioxide to be intermediate in expired air when compared to inspired air and alveolar air. Some oxygen has been lost to the alveolus, lowering its level to 120, carbon dioxide has been gained from the alveolus raising its level to 27.

Likewise a concentration gradient causes oxygen to diffuse into the blood from the alveoli and carbon dioxide to leave the blood. This produces the levels seen in oxygenated blood in the body. When this blood reaches the systemic tissues the reverse process occurs restoring levels seen in deoxygenated blood.

The thyroid gland is a double-lobed structure located in the neck. Embedded in its rear surface are the four parathyroid glands.

The Thyroid Gland

The thyroid gland synthesizes and secretes:

• thyroxine (T₄) and
• calcitonin

T₄ and T₃

Thyroxine (T₄ ) is a derivative of the amino acid tyrosine with four atoms of iodine. In the liver, one atom of iodine is removed from T₄ converting it into triiodothyronine (T₃). T₃ is the active hormone. It has many effects on the body. Among the most prominent of these are:

• an increase in metabolic rate
• an increase in the rate and strength of the heart beat.

The thyroid cells responsible for the synthesis of T₄ take up circulating iodine from the blood. This action, as well as the synthesis of the hormones, is stimulated by the binding of TSH to transmembrane receptors at the cell surface.

Diseases of the thyroid

1. hypothyroid diseases; caused by inadequate production of T₃

• cretinism: hypothyroidism in infancy and childhood leads to stunted growth and intelligence. Can be corrected by giving thyroxine if started early enough.
• myxedema: hypothyroidism in adults leads to lowered metabolic rate and vigor. Corrected by giving thyroxine.
• goiter: enlargement of the thyroid gland. Can be caused by:
  • inadequate iodine in the diet with resulting low levels of T₄ and T₃;
  • an autoimmune attack against components of the thyroid gland (called Hashimoto's thyroiditis).

2. hyperthyroid diseases; caused by excessive secretion of thyroid hormones

Graves´ disease. Autoantibodies against the TSH receptor bind to the receptor mimicking the effect of TSH binding. Result: excessive production of thyroid hormones. Graves´ disease is an example of an autoimmune disease.

Osteoporosis. High levels of thyroid hormones suppress the production of TSH through the negative-feedback mechanism mentioned above. The resulting low level of TSH causes an increase in the numbers of bone-reabsorbing osteoclasts resulting in osteoporosis.

Calcitonin

Calcitonin is a polypeptide of 32 amino acids. The thyroid cells in which it is synthesized have receptors that bind calcium ions (Ca²⁺) circulating in the blood. These cells monitor the level of circulating Ca²⁺. A rise in its level stimulates the cells to release calcitonin.

• bone cells respond by removing Ca²⁺ from the blood and storing it in the bone
• kidney cells respond by increasing the excretion of Ca²⁺

Both types of cells have surface receptors for calcitonin.

Because it promotes the transfer of Ca²⁺ to bones, calcitonin has been examined as a possible treatment for osteoporosis

• it's the individual pressure exerted independently by a particular gas within a mixture of gasses. The air we breath is a mixture of gasses: primarily nitrogen, oxygen, & carbon dioxide. So, the air you blow into a balloon creates pressure that causes the balloon to expand (& this pressure is generated as all the molecules of nitrogen, oxygen, & carbon dioxide move about & collide with the walls of the balloon). However, the total pressure generated by the air is due in part to nitrogen, in part to oxygen, & in part to carbon dioxide. That part of the total pressure generated by oxygen is the 'partial pressure' of oxygen, while that generated by carbon dioxide is the 'partial pressure' of carbon dioxide. A gas's partial pressure, therefore, is a measure of how much of that gas is present (e.g., in the blood or alveoli). 
   
• the partial pressure exerted by each gas in a mixture equals the total pressure times the fractional composition of the gas in the mixture. So, given that total atmospheric pressure (at sea level) is about 760 mm Hg and, further, that air is about 21% oxygen, then the partial pressure of oxygen in the air is 0.21 times 760 mm Hg or 160 mm Hg.

Hormones are carried by the blood throughout the entire body, yet they affect only certain cells.  The specific cells that respond to a given hormone have receptor sites for that hormone.  

This is sort of a lock and key mechanism.  If the key fits the lock, then the door will open.  If a hormone fits the receptor site, then there will be an effect.  If a hormone and a receptor site do not match, then there is no reaction.  All of the cells that have receptor sites for a given hormone make up the target tissue for that hormone.  In some cases, the target tissue is localized in a single gland or organ.  In other cases, the target tissue is diffuse and scattered throughout the body so that many areas are affected.  

Hormones bring about their characteristic effects on target cells by modifying cellular activity.  Cells in a target tissue have receptor sites for specific hormones.  Receptor sites may be located on the surface of the cell membrane or in the interior of the cell.

In general those protein hormones are unable to diffuse through the cell membrane and react with receptor sites on the surface of the cell.  The hormone receptor reaction on the cell membrane activates an enzyme within the membrane, called adenyl cyclase, which diffuses into the cytoplasm.  Within the cell, adenyl cyclase catalyzes or starts the process of removal of phosphates from ATP to produce cyclic adenosine monophosphate or c AMP.  This c AMP activates enzymes within the cytoplasm that alter or change the cellular activity.  The protein hormone, which reacts at the cell membrane, is called the first messenger.  c Amp that brings about the action attributed to the hormone is called the second messenger.  This type of action is relatively rapid because the precursors are already present and they just needed to be activated in some way.  

Neurons :

Types of neurons based on structure:

a multipolar neuron because it has many poles or processes, the dendrites and the axon. Multipolar neurons are found as motor neurons and interneurons. There are also bipolar neurons with two processes, a dendrite and an axon, and unipolar neurons, which have only one process, classified as an axon.. Unipolar neurons are found as most of the body's sensory neurons. Their dendrites are the exposed branches connected to receptors, the axon carries the action potential in to the central nervous system.

Types of neurons based on function:

• motor neurons - these carry a message to a muscle, gland, or other effector. They are said to be efferent, i.e. they carry the message away from the central nervous system.
• sensory neurons - these carry a message in to the CNS. They are afferent, i.e. going toward the brain or spinal cord.
• interneuron (ie. association neuron, connecting neuron) - these neurons connect one neuron with another. For example in many reflexes interneurons connect the sensory neurons with the motor neurons.